Since vinyl alcohol-based polymers (hereinafter, may be abbreviated as PVA) have excellent interfacial properties and strength properties as one of the few crystalline water soluble polymers, they are utilized in various binders, paper processing, fiber processing, stabilizers for emulsion and also occupies an important position as a material for PVA-based films, PVA-based fibers, and the like. Meanwhile, they are also sought for higher functionality to improve specific performances by controlling the crystallinity or introducing functional groups, and a variety of so-called modified PVAs are developed as well.
Among all, a variety of PVAs having a polyoxyalkylene group (hereinafter, may be abbreviated as a POA group) are developed. For example, in Patent Document 1, a method of synthesizing PVA having a POA group is disclosed that is carried out by saponifying a copolymer of a vinyl ester and polyoxyalkylene (hereinafter, may be abbreviated as POA) having an allyl group at an end, a methacrylic ester having a POA group, or a methacrylamide group containing compound having a POA group on a nitrogen atom, and after that PVAs having a POA group have come to be utilized for a variety of applications. For example, its utility as a dispersion stabilizer in Patent Document 2 and its utility as a resin for melt molding in Patent Document 3 are disclosed respectively.
Most of the PVA having a POA group that have been utilized in the past have a POA group with an alkylene unit of ethylene, and such principles have been utilized that a polyoxyethylene group imparts interfacial properties and strength properties to PVA and suppresses crystallinity of PVA.
Meanwhile, Patent Document 4 discloses that an aqueous solution of PVA having a polyoxypropylene group as a POA group, in a case of a certain number or more of oxyalkylene repeating units, has a high viscosity and also expresses a rise in viscosity accompanied by a rise in temperature. Side chain of polyoxypropylene modified PVA is more hydrophobic compared with side chain of polyoxyethylene modified PVA, and the intermolecular interaction of polyoxypropylene with each other functions in water, and thereby such physical properties mentioned above are considered to be obtained.
However, since the intermolecular interaction of polyoxypropylene with each other is not so strong in an aqueous solution of PVA having a polyoxypropylene group, such an approach of increasing the POA group content or increasing the concentration of the aqueous PVA solution is necessary to obtain a highly viscous aqueous PVA solution, and POA modified PVA which provides high viscosity of the aqueous PVA solution with a lower content has been demanded.
Meanwhile, making use of the synthesis method described in Patent Document 1, for example, a film for packaging alkaline substances and a flexible water resistant film in which heat treatment of the film is devised are proposed respectively in Patent Document 5 and Patent Document 6.
Here, most of the polyoxyalkylene groups disclosed in Patent Documents 1, 5, and 6 are a polyoxyethylene group and polyoxypropylene is disclosed only partly. This signifies that, since these Patent Documents are intended to obtain a polymer or a film having excellent water solubility at low temperatures, ethylene or propylene, which has a less carbon number, is selected as alkylene.
However, although the PVA containing a polyoxyethylene group or a polyoxypropylene group and films thereof disclosed in Patent Documents 1, 5, and 6 are excellent in water solubility at low temperatures, they have problems of easy absorption of moisture and a serious decrease in the elastic modulus of the films while leaving the films in high humidity. While they are excellent in water solubility, they have high affinity for water, so that they have a problem that they are low in the water repellency when water droplets are attached on the film surface, for example, and the attached portions shrink easily to be prone to become a defect.